1. Field of the Invention
This invention relates generally to automatic transmissions for automotive vehicles, in particular to transmissions comprising planetary gearsets operated by friction control elements.
2. Description of the Prior Art
It is difficult to achieve acceptable shift quality on sequential, i.e., continuous downshifts, such as a 6-4-3 or 5-3-2 downshift, in an automatic transmission because a torque disturbance may occur during the transition from the first to the second portion of the shift. In order to execute the transition smoothly, the offgoing control element, a clutch or brake, for the second shift must slip before the oncoming element of the first shift gains torque capacity.
The shifts are difficult to calibrate robustly. If the offgoing element slips too soon, a neutral interval occurs near the end of the shift. Conversely, if the offgoing element slips too late, a torque bump occurs as the oncoming element of the first shift gains capacity.
To achieve acceptable feel, the oncoming and offgoing elements must be closely synchronized. But precise synchronization is difficult to achieve under all operating conditions.
The period required to execute successive downshifts between adjacent gears using conventional control techniques is unacceptable approximating 1.2 seconds to complete such downshifts. There is a need in the industry for a control strategy that permits successive downshifts to be completed smoothly and within an acceptably short period.